This invention relates to a digital data and orderwire combiner apparatus particularly adapted to operate with a fiber optic information transmission system.
Fiber optic systems using glass fiber guides are extensively employed as communication means. In regard to such systems as for example a communications system, the objective is to transfer information from one point to another point which points or terminals are remotely located. The transfer of information is accomplished by modulating the information onto a carrier wave which is then transmitted or propagated to the receiving sight where the information is recovered by a demodulation process.
There are many advantages in communicating with optical frequencies in regard to an increase in information that can be transmitted over an optical link plus lower power dissipation plus more reliable transmissions in regard to RF noise and so on. Thus the field of optical communications has been rapidly developing. In regard to this, optical communication systems are widely employed both in industrial and military use.
In regard to military applications, most communications equipment used by the military possesses a similar signal format. This information consists of a data signal where the data rate can be from 72,000 BPS to 2.304 MBPS. The data signal is linearly added to an orderwire signal. Orderwire is defined as a low speed data or voice channel used for maintenance or setup purposes.
As one can ascertain from the prior art, there are many types of orderwire signals. There is a digital signal which is known as system orderwire or Combined Voice Digital Orderwire and sometimes referred to as a CVDOW signal. There are also analog orderwire signals.
In any event, one requires combiners which esssentially operate reliably to transmit the combined or traffic signal over a cable or to apply the signal to a suitable laser diode for subsequent transmission over fiber optic link. The circuit that performs the data combination with the orderwire signal must be reliable in operation and must exhibit excellent operating properties over wide temperature ranges while avoiding impressing undue jitter or phase delays in regard to the combined signal.
Such considerations require a circuit or a digital data orderwire signal combiner at the transmission end which circuit functions to cancel many offset errors which are inherent in prior art circuits while minimizing signal jitter due to the orderwire which is conventionally at a lower frequency than the data signal. As one can ascertain, a typical optical communication system has one or more data sources where the output is modulated onto an optical carrier. The carrier is then transmitted as an optical light field or a beam through the optical channel which is for example an optical fiber. At the receiver, the field is optically collected and processed by the use of a photodetector or other device.
In regard to such systems, the source for light transmission includes semiconductor lasers and LED devices. LED's are adequate for data links and are extensively used in the field of optical communications. As such, longer wavelength LED's are also employed for major transmission roles. In any event, there is a problem with LED devices in that they exhibit non-linearities in operation particularly when they are operated at high current levels to produce high power outputs. Thus the circuit which operates to drive the LED must be capable of driving the relatively low impedance device with sufficient amplitude and linearity. the circuit must also possess good stability over wide temperature ranges while exhibiting low jitter so as not to produce undue interference in the transmitted signal.
It is therefore and object of the present invention to provide an improved digital data orderwire signal combiner which is utilized in a fiber optic communications link at a transmitting end.
It is further object of the present invention to provide a data and orderwire combiner which is reliable over a wide temperature range and is capable of driving a cable or a LED device in a reliable and consistent manner.